1. Field of the Invention
The present invention relates to a trench MOSFET and, more particularly, to a structure of a trench MOSFET with improved thermal conduction and a method for manufacturing the same.
2. Description of the Prior Art
In the structure of a trench Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) or vertical transistor, the gate of the transistor is formed in a trench on top of a substrate, and the source/drain regions of the transistor are formed on both sides of the gate. This type of vertical transistor allows high current to flow therethrough and turns on/off at a low gate voltage.
Referring to FIG. 1, a cross-sectional diagram of the structure of a trench MOSFET is shown. An N-type doping epitaxial region 105 is provided on an N+substrate 100. A plurality of trenches are formed in the region 105. The region 105 has a lower doping concentration than the substrate 100. A gate oxide layer 115 and gates 130 are covered in the trenches. P-type doping regions (hereinafter call P-body) 110 are formed on both sides of the gates 130. N+doping regions 125 are formed in the P-body 110 in active regions used as the source regions of the transistor. Metal connections for the gates 130, P-body 110 and N+doping regions 125 are formed from tungsten metal plugs 145 and barrier layers 140 in an insulating layer 135. An aluminum alloy metal layer 150 above the tungsten metal plug 145 is used as a second layer of metal connection. However, using the aluminum alloy metal plug as the metal connections in the trench MOSFET may cause poor thermal conduction as the size of the transistor shrinks.
Prior arts (U.S. Pat. Nos. 6,462,376 and 6,888,196) have 2-dimensional source contact with a tungsten plug which is connected with aluminum alloys as a front metal. The metal system has a thermal conduction issue when the die size shrinks as a result of increasing cell density.
The present invention provides a new structure of trench MOSFET, which uses copper as a front metal structure and which has better contact and thermal conduction than the prior art.